Cdk9 is a key elongation factor for RNA transcription and functions by phosphorylating the C-terminal domain of RNA polymerase II. Here we present direct evidence that cdk9 is important for cancer cell survival and describe the characterization of the potent cdk9 inhibitor CDKI-73 in primary human leukemia cells. CDKI-73 induced caspase-dependent apoptosis that was preceded by dephosphorylation of cdk9 and serine 2 of RNA polymerase II. CDKI-73 was more potent than the pan-cdk inhibitor flavopiridol and showed >200-fold selectivity against primary leukemia cells when compared with normal CD34+ cells. Furthermore, CDKI-73 was equipotent in poor prognostic sub-groups of leukemia patients and showed cytotoxic synergy with the nucleoside analog fludarabine. The Mechanism of synergy was associated with CDKI-73-mediated transcriptional inhibition of MCL1 and XIAP that was maintained when used in combination with fludarabine. Our data present a strong rationale for the development of cdk9 inhibitors such as CDKI-73 as anticancer therapeutics.

BackgroundAurora kinases play an essential role in the orchestration of chromosome separation and cytokinesis during mitosis. Small-molecule inhibition of the aurora kinases has been shown to result in inhibition of cell division, phosphorylation of histone H3 and the induction of apoptosis in a number of cell systems. These characteristics have led aurora kinase inhibitors to be considered as potential therapeutic agents.

BackgroundTopoisomerase II is essential for the maintenance of DNA integrity and the survival of proliferating cells. Topoisomerase II poisons, including etoposide and doxorubicin, inhibit enzymemediated DNA ligation causing the accumulation of double-stranded breaks and have been front-line drugs for the treatment of leukemia for many years. Voreloxin is a first-in-class anticancer quinolone derivative that intercalates DNA and inhibits topoisomerase II. The efficacy and mechanisms of action of voreloxin in acute myeloid leukaemia were addressed in this study. Design and MethodsPrimary acute myeloid leukemia blasts (n = 88) and myeloid cell lines were used in vitro to study voreloxin through viability assays to assess cell killing and synergy with other drugs. Apoptosis and cell cycling were assessed by flow cytometry. DNA relaxation assays were utilized to determine that voreloxin was active on topoisomerase II. ResultsThe mean lethal dose 50% (LD50) (± standard deviation) of voreloxin for primary acute myeloid leukemia blasts was 2.30 mM (± 1.87). Synergy experiments between voreloxin and cytarabine identified synergism in 22 of 25 primary acute myeloid leukemia samples tested, with a mean combination index of 0.79. Apoptosis was shown to increase in a dose-dependent manner. Furthermore, voreloxin was active in the p53-null K562 cell line suggesting that the action of voreloxin is not affected by p53 status. The action of voreloxin on topoisomerase II was confirmed using a DNA relaxation assay. ConclusionsVoreloxin may provide an interesting addition to the cache of drugs available for the treatment of acute myeloid leukemia, a disease with a poor long-term survival. In addition to its potent action as a single agent in dividing cells, the synergy we demonstrated between voreloxin and cytarabine recommends further investigation of this topoisomerase II inhibitor.Key words: voreloxin, topoisomerase II inhibitor, myeloid leukemia, synergism, cytarabine. cytarabine. Haematologica 2011;96(3):393-399. doi:10.3324/haematol.2010 This is an open-access paper. Citation: Walsby EJ, Coles SJ, Knapper S, and Burnett AK. The topoisomerase II inhibitor voreloxin causes cell cycle arrest and apoptosis in myeloid leukemia cells and acts in synergy with

Key Points• LN-derived CLL cells have increased capacity for T-cell activation and superior immune synapse formation compared with those from PB.• Enhanced CLL cell immunologic function is also linked to PB circulating cells with the propensity to migrate.Several lines of evidence suggest that homing of tumor cells to lymphoid tissue contributes to disease progression in chronic lymphocytic leukemia (CLL). Here, we demonstrate that lymph node (LN)-derived CLL cells possess a distinct phenotype, and exhibit enhanced capacity for T-cell activation and superior immune synapse formation when compared with paired peripheral blood (PB) samples. LN-derived CLL cells manifest a proliferative, CXCR4 dim CD5 bright phenotype compared with those in the PB and higher expression of T-cell activation molecules including CD80, CD86, and HLA-D-related (DR). In addition, LN-CLL cells have higher expression of a4b1 (CD49d) which, as well as being a co-stimulatory molecule, is required for CLL cells to undergo transendothelial migration (TEM) and enter the proliferation centers of the LNs. Using an in vitro system that models circulation and TEM, we showed that the small population of CLL cells that migrate are CXCR4 dim CD5 bright with higher CD49d, CD80, CD86, and HLA-DR compared with those that remain circulating; a phenotype strikingly similar to LN-derived CLL cells. Furthermore, sorted CD49d hi CLL cells showed an enhanced capacity to activate T cells compared with CD49d lo subpopulations from the same patient. Thus, although PB-CLL cells have a reduced capacity to form immune synapses and activate CD4 1 T cells, this was not the case for LN-CLL cells or those with the propensity to undergo TEM. Taken together, our study suggests that CLL cell immunologic function is not only modulated by microenvironmental interactions but is also a feature of a subpopulation of PB-CLL cells that are primed for lymphoid tissue homing and interaction with T cells. (Blood. 2016;128(4):563-573)

2'-Beta-D-arabinouridine (AraU), the uridine analogue of the anticancer agent AraC, was synthesized and evaluated for antiviral activity and cytotoxicity. In addition, a series of AraU monophosphate prodrugs in the form of triester phosphoramidates (ProTides) were also synthesized and tested against a range of viruses, leukaemia and solid tumour cell lines. Unfortunately, neither the parent compound (AraU) nor any of its ProTides showed antiviral activity, nor potent inhibitory activity against any of the cancer cell lines. Therefore, the metabolism of AraU phosphoramidates to release AraU monophosphate was investigated. The results showed carboxypeptidase Y, hog liver esterase and crude CEM tumor cell extracts to hydrolyse the ester motif of phosphoramidates with subsequent loss of the aryl group, while molecular modelling studies suggested that the AraU l-alanine aminoacyl phosphate derivative might not be a good substrate for the phosphoramidase enzyme Hint-1. These findings are in agreement with the observed disappearance of intact prodrug and concomitant appearance of the corresponding phosphoramidate intermediate derivative in CEM cell extracts without measurable formation of araU monophosphate. These findings may explain the poor antiviral/cytostatic potential of the prodrugs.